UI Text is Blurry in RenderTexture

I use a Camera to render what it sees, there are some text labels in the view of the Camera, then put the rendered result into a renderTexture, I found the texts are blurry when I show the renderTexure, but they are clear and sharp when I check them directly by the Camera.

I didn’t find anyone who issued the similar problem, is my unity version lead to that? my unity version: 2020.1.0a25.

first image is what renderTexture shows.

second image is what the Camera shows.

Anyone can help me with this problem?